1. Field of the Invention
This invention relates to a foreign particle inspecting apparatus, a foreign particle inspecting method and a foreign particle data indicating method, and particularly to a foreign particle inspecting apparatus, a foreign particle inspecting method and a foreign particle data indicating method for foreign particles on a substrate such as a reticle or a photo-mask used in a semiconductor manufacturing process or a liquid crystal display element manufacturing process.
2. Related Background Art
In manufacturing a semiconductive integrated circuit or a liquid crystal display element by the use of the photography technique, use is made of a reticle or a photo-mask (hereinafter generically referred to as the "reticle") on which is formed a pattern to be transferred onto a wafer having photo-resist applied thereto. If in the manufacturing process, a foreign particle such as dust adheres to the pattern area of the reticle, there will occur a defect common to all chips of the wafer onto which the pattern of the reticle is transferred. Therefore, in the manufacturing process, it is necessary to strictly inspect the presence or absence of any foreign particle on the pattern area of the reticle. Thus, as an apparatus for automatically inspecting any foreign particle adhering to the surface of an object to be inspected, use has heretofore been made of a foreign particle inspecting apparatus which two-dimensionally scans the surface of the object to be inspected by a light beam from a predetermined light source, and detects scattered light from a foreign particle on that surface to thereby detect the foreign particle. Such an apparatus is disclosed in U.S. Pat. No. 4,468,120.
Also, in recent times, there are cases where a light transmitting thin film (hereinafter referred to as the "pellicle") is mounted on the surface of the reticle through a support frame to reduce the influence of any foreign particle. The pellicle is for preventing the adherence of any foreign particle onto the surface of the reticle. Again for the reticle thus covered with the pellicle, it is necessary to inspect the presence or absence of any foreign particle on the reticle because a foreign particle or particles may adhere to the surface of the reticle during the mounting of the pellicle.
However, with the pellicle mounted on the reticle, the light beam cannot be applied to the reticle from a low position due to the presence of the support frame for the pellicle and further, scattered light from a foreign particle cannot be received at a low angle. Therefore, with the pellicle remaining mounted on the reticle, foreign particle inspection cannot be accomplished well. In this regard, a foreign particle inspecting apparatus which can well accomplish the detection of any foreign particle on a reticle on which a pellicle is mounted is disclosed in Japanese Laid-Open Patent Application No. 63-118640.
Also, when foreign particle detection is to be effected for a surface to be inspected on which a pellicle is mounted, inspecting light is caused to impinge on the surface to be inspected through the pellicle and scattered light from any foreign particle on the surface to be inspected enters a detecting optical system again through the pellicle. When the transmittance of the pellicle for incident light is Ti (hereinafter referred to as the "incident light transmittance Ti") and the transmittance of the pellicle for the scattered light from the foreign particle is Ts (hereinafter referred to as the "scattered light transmittance Ts"), to equally discriminate between the size of the foreign particle on the surface to be inspected on which the pellicle is mounted and the same degree of size of a foreign particle on a surface to be inspected on which no pellicle is mounted, it is necessary that the detection signal by the foreign particle on the surface to be inspected on which the pellicle is mounted be corrected to 1/Ti.multidot.Ts times.
Also, generally, the angle of incidence of a light beam onto the pellicle varies when the scanning position of the light beam on the surface of the reticle to be inspected varies. Also, when the scanning position of the light beam Varies, the angle at which the scattered light from the foreign particle, scattered light travelling toward a detector, is incident on the pellicle, in other words, the angle of incidence of a detection optical axis determined by the positional relationship between the scanning position of the light beam and the detecting optical system (hereinafter referred to as the "angle of emergence"), varies. Therefore, by the variation in the .angle of incidence, the transmittance of the pellicle for the light beam is varied, and by the variation in the angle of emergence, the transmittance of the pellicle for the scattered light from the foreign particle is varied. The detection sensitivity for the foreign particle is also varied by the variation in the transmittance of the pellicle for the light beam, and the detection sensitivity for the foreign particle is also varied by the variation in the transmittance of the pellicle for the scattered light from the foreign particle. Particularly when the thickness of the pellicle is of the order of the wavelength of the light beam and the light beam is monochromatic light, the wavelength selectivity by the thickness of the pellicle is added, and the transmittance of the pellicle for the light beam is greatly varied by the variation in the angle of incidence, and the transmittance of the pellicle for the scattered light from the foreign particle is greatly varied by the variation in the angle of emergence. In other words, even for a foreign particle of the same degree of size on the same surface to be inspected on which the pellicle is mounted, if it adheres to a different scanning position, the magnitude of the detection signal will differ.
So, U.S. Pat. No. 4,889,998 discloses a method of predetermining the transmittance of a pellicle for a light beam and the transmittance of the pellicle for scattered light from a foreign particle, with respect to each scanning position on a surface to be inspected, and correcting the magnitude of a detection signal in conformity with the scanning position of the light beam, and discloses a foreign particle inspecting apparatus for correcting any variation in the detection sensitivity for the foreign particle attributable to a variation in the transmittance of the pellicle for the light beam.
In the foreign particle inspecting apparatus, a laser beam is often used as a light source, and generally the laser beam used often has a particular direction of polarization.
Also, by making the direction of polarization of a polarized beam particular relative to a surface to be inspected, the detection signal by a foreign particle can be intensified relative to the detection signal by a circuit pattern on the surface to be inspected and therefore, in some cases, a polarized laser is positively used for foreign particle inspection with a view to make the presence of a foreign particle on the surface to be inspected conspicuous.
As described above, in the foreign particle inspecting apparatus, foreign particle inspection is often done by the use of a polarized beam, and in the apparatus disclosed in U.S. Pat. No. 4,889,998, the transmittance measured with inspecting light (a polarized beam) caused to impinge on a pellicle is regarded as the transmittance of scattered light from a foreign particle and the magnitude of the detection signal of the scatterd light is corrected.
However, even when a foreign particle is irradiated with a polarized beam, the scattered light by the irregular reflecting surface of the foreign particle has its polarized state destroyed, and the transmittance of the pellicle for the polarized beam at the same angle of incidence (the angle of incidence and the angle of emergence are the same) and the transmittance of the pellicle for the scattered light from the foreign particle whose polarized state is destroyed differ greatly from each other. Accordingly, in the prior-art foreign particle inspecting apparatus disclosed in U.S. Pat. No. 4,889,998, the error of the magnitude of the detection signal corresponding to the size of the foreign particle on the surface to be inspected is enlarged (any variation in the detection sensitivity for the foreign particle by the influence of the transmittance of the pellicle for the scattered light becomes unable to be accurately detected) and it becomes impossible to equally discriminate between the size of the foreign particle on the surface to be inspected on which the pellicle is mounted and the same degree of size of a foreign particle on a surface to be inspected on which no pellicle is mounted, or to accurately detect the same degree of foreign particle adhering to a different scanning position on the same surface to be inspected.
Further, the prior-art foreign particle inspecting apparatus has the following inconvenience.
Heretofore, the correction of foreign particle detection sensitivity has been effected by the use of the data of the pellicle transmittance preknown for the same pellicles, but even for the same pellicles, the transmittance differs greatly in some cases due to the difference in film thickness within the manufacturing tolerance. Therefore, it is necessary to calculate the transmittance by the use of the actual film thickness data of each pellicle, in addition to the average transmittance data of the pellicles, but it will increase the burden of the pellicle manufacturer to demand the actual film thickness data of each pellicle of the pellicle manufacturer, and it is not always possible to accurately calculate the transmittance of each pellicle. Accordingly, it is not realistic to pre-calculate the data of the transmittance of each pellicle.
In contrast, there has also been proposed a method of providing a transmittance measuring portion for actually measuring the transmittance of each pellicle, discretely from a foreign particle inspecting portion. In this case, it is difficult to measure the transmittance of the pellicle by just the same system as the foreign particle inspecting portion and generally, a system for measuring the transmittance by a simplified construction is incorporated as the transmittance measuring portion. However, in some cases, a correction error of transmittance arises from the difference in construction between the foreign particle inspecting portion and the transmittance measuring portion, but no consideration has heretofore been given to such a correction error.
Also, in the prior-art foreign particle inspecting apparatus, it is generally practiced to map-display the result of the detection of foreign particles (the number and sizes of foreign particles).
Specifically, as shown, for example, in FIG. 16 of the accompanying drawings, display has been made divisionally for each magnitude of the number (rank) of foreign particles. Particularly in the map display, the largest foreign particle has been displayed when a plurality of foreign particles exist within one and the same section. In FIG. 16, a map 4m indicates the locations and sizes of detected foreign particles. The size of the sections in the map 4m indicates sections of a predetermined size (e.g. 1 mm-several mm square) on the surface to be inspected, and the size of the largest foreign particle in these sections is rank-displayed. In the map 4m of FIG. 16, ranks A, B and C are displayed in the order of the magnitudes of ranks, and it is indicated that a foreign particle of rank A and two foreign particles of rank C have been detected. Further, in FIG. 16, the numbers of foreign particles within the display screen are indicated by ranks below the map 4m. In such a conventional indicating method, when a plurality of foreign particles exist within one and the same section, only the largest foreign particle is indicated. Therefore, the other foreign particles (foreign particles smaller than the indicated foreign particle) existing within the same section as the indicated foreign particle are not indicated, and this has led to the problem that the presence of small foreign particles cannot be confirmed. If an attempt is made to confirm the presence of such foreign particles which are not indicated, map display can be made with the size of a unit section area made smaller, but this has led to the problem that only a part of the area on the surface to be inspected is indicated or the map becomes larger. There is also a method of switching the map display for the confirmation of small foreign particles, but it takes time and labor to switch the map display each time small foreign particles are confirmed, and this is not practical.